mc33975 Freescale Semiconductor, Inc, mc33975 Datasheet - Page 24

• Manufacturer Part Number: mc33975
• Description: Multiple Switch Detection Interface With Suppressed Wake-up And 32ma Wetting Current
• Manufacturer: Freescale Semiconductor, Inc

closed switch contacts. However, there are many features
that allow the device to be used in a variety of applications.
The following is a list of applications to consider for the IC:
detail.
SENSOR POWER SUPPLY
external to a module. Many sensors such as Hall effect,
pressure sensors, and temperature sensors require a supply
voltage to power the sensor, and provide an open collector or
analog output.
to supply power and interface to these types of sensors. In an
application where the input makes continuous transitions,
consider using the wake-up/interrupt command to disable
the interrupt for the particular input.
24
33975
TYPICAL APPLICATIONS
OPERATIONAL MODES
Reg
X
The 33975’s primary function is the detection of open or
• Sensor power supply
• Switch monitor for metallic or elastomeric switches
• Analog sensor inputs (Ratiometric)
• Power MOSFET/LED driver and monitor
• Multiple 33975 devices in a module system
The following paragraphs describe the applications in
Each input may be used to supply current to sensors
Hall-Effect
Sensor
VBAT
VBAT
32 mA
V
0V
PWR
SG12
SG13
SG0
SG1
Figure 13. Sensor Power Supply
SP7
SP0
SP1
Figure 13
SG13
32
mA
32
mA
V
V
PWR
PWR
V
V
33975
shows how the 33975 may be used
PWR
PWR
4.0
mA
4.0
mA
WAKE
VPWR
AMUX
SCLK
V
0V
VDD
DD
INT
SO
CS
SI
V
AMUX
DD
TYPICAL APPLICATIONS
MOSI
SCLK
CS
MISO
INT
IOC[7:0]
Input Capture
Timer Port
MCU
METALLIC/ELASTOMERIC SWITCH
resistance over time owing to contact corrosion. The
corrosion is induced by humidity, salt, and other elements
that exist in the environment. For this reason the 33975
provides two settings for contacts. When programmed for
metallic switches, the device provides higher wetting current
to keep switch contacts free of oxides. The higher current
occurs for the first 20ms of switch closure. Where longer
duration of wetting current is desired, the user may send the
wetting current timer command and disable the timer. Wetting
current will be continuous to the closed switch. After the time
period set by the MCU, the wetting current timer command
may be sent again to enable the timer. The user must
consider power dissipation on the device when disabling the
timer. (Refer to the paragraph entitled
page 23.)
contact, the user has two options. Higher wetting current to a
switch may be achieved by paralleling SGn or SPn inputs.
This will increase wetting current by 32mA for each input
added to the switch-to-ground contact and 16mA for switch-
to-battery contacts. The second option is to simply add an
external resistor pull-up to the V
ground inputs or a resistor to ground for a switch-to-battery
input. Adding an external resistor has no effect on the
operation of the device.
a high contact resistance. Resistance of 1.0 kΩ is common.
In applications with elastomeric switches, the pull-up and
pull-down currents must be reduced to prevent excessive
power dissipation at the contact. Programming for a lower
current settings is provided in the
Operation
Metallic Command.
ANALOG SENSOR INPUTS (RATIOMETRIC)
the binary code for a specific input in the analog command
allows the microcontroller to perform analog to digital
conversion on any of the 22 inputs. On rising edge of the
the multiplexer connects a requested input to the AMUX pin.
The AMUX pin is clamped to max of VDD volts regardless of
the higher voltages present on the input pin. After an input
has been selected as the analog, the corresponding bit in the
next SO data stream will be logic [0].
configured as analog with high-impedance, analog with
4.0mA pull-up, or analog with 32mA pull-up.
page 25, shows how the 33975 may be used to provide a
ratiometric reading of variable resistive input.
Metallic switch contacts often develop higher contact
To increase the amount of wetting current for a switch
Elastomeric switch contacts are made of carbon and have
The 33975 features a 22-to-1 analog multiplexer. Setting
The input pin, when selected as analog, may be
Section beginning on page
Analog Integrated Circuit Device Data
PWR
Functional Device
Freescale Semiconductor
supply for switch-to-
16
Temperature
under
Figure
Table
14,
Monitor,
8,
CS,